The hall was booked and the invitations sent out. Mary Ann Sieghart and her husband David had been looking forward to their joint birthday party for months.

But as the day drew nearer they started to dread the event. Though many of the 200 guests had been friends or colleagues for years, Mary Ann knew she had little chance of recognising anyone except her immediate family.

And she couldn't rely on husband David for help as, incredibly, he also suffers from the same rare condition that means they can't distinguish one person from another by their faces.

Prosopagnosis, which means they are both face blind, has also hit the couple's daughter Evie, 16, and Mary Ann's mum, Felicity Ann. Their other daughter, Rosa, 14, is unaffected.

Remarkably, Mary Ann even held down a high-powered job on a national newspaper.

She says: "It's a great source of social embarrassment as I just can't remember if I know that person and if I do, where I might know them from.

"Of course, we knew everybody who was coming to our party but, out of context, we knew we'd have no chance of working out who they were and we couldn't even help each other."

The couple eventually spent most of the night last August trying to memorise what each guest was wearing so they could remember who was who for the evening.

"I can usually cope by bluffing my way through but, of course, with so many people that was always going to be difficult," says David, 55.

And it's typical of the way the condition affects the family's daily lives. "We've always been useless at parties and usually spend the whole evening whispering 'who was that?' to each other so you can imagine how nervous we were holding our own," says Mary Ann, 46.

"My daughter even joked that we should all have T-shirts saying 'Don't blame me, I'm prosopagnosic' to get us out of tricky social situations.

"It's awful when people think you're being rude by not recognising them even though you might see them every day."

Mary Ann first became aware of it when she was eight. Reading her favourite Enid Blyton adventures she was amazed by the way the children were able to give such accurate descriptions of the baddies to the police.

"I remember thinking I wouldn't know where to start and I certainly wouldn't be able to recognise them," she says.

Her mother had also been terrible with faces and the pair often joked that Mary Ann must take after her.

Then, as a teenager watching movies, Mary Ann realised she was struggling to keep up with the plots because she couldn't tell one character from another.

She says: "Me and my brother watched a film with Steve McQueen and Paul Newman.

When it had finished he asked me which character was which and I had to confess that I didn't have a clue.

"Both were good-looking with blue eyes so there was nothing to help me tell one from the other."

Again, Mary Ann and her family just put it down to her being bad with faces, the way some people are with names. But things got even harder when she went to university.

Each day brought a sea of new faces and Mary Ann was constantly apologising for not knowing people, even though she'd already met them several times.

"I felt so guilty for having to keep asking somebody their name and who they were when they clearly already knew me," she says. "Some people thought I was lazy or uninterested but nothing could have been further from the truth."

Slowly she started to remember the names of her friends, relying on things like the colour of their bag, the length of their hair and the style of their glasses. Of course, that meant she'd be back to square one again if that person changed their appearance. "If a friend had their hair cut I could easily pass them in the street and not have a clue who they were," she says.

It was even harder if a person had symmetrical features, as a big nose or wonky ears helped trigger her recognition. So Mary Ann developed strategies that would help her learn a person's name without having to offend them by asking again. "If I was standing with one person I couldn't remember and then another approached that I also didn't recognise I would invite them to introduce themselves to each other, which would give me both their names."

In 1986 Mary Ann was introduced to David. One of the many things they had in common was that he was also "bad with faces".

"I'd always thought I just had a bad memory," explains David. And when they married in 1989, David joked that they should ask their guests to wear name badges.

The couple couldn't even do normal things like watch films - Brad Pitt and Tom Cruise look identical to them.

Even when their eldest daughter Evie started to show signs of being unable to recognise patterns aged seven, the couple still assumed it was just one of those things. Mary Ann's mum had been the same and now it seemed their daughter would be too.

Her teacher suggested Evie see a educational psychologist who was shocked when she struggled to put together a six-piece puzzle of a human face. But still she wasn't diagnosed.

She had problems recognising friends, too. "Once she told me she'd met a nice friend, she didn't know her name or what she looked like, only that she wore a red jumper," says Mary Ann.

"I had to point out that the girl might not wear a red jumper every day so Evie had to work out another way of recognising her."

Then in July last year Mary Ann read an article about prosopagnosis.

Amazed, she realised she had all of the symptoms, as did her husband, daughter and mother.

"I was so excited. I knew instantly that there was no question I had this condition. I wasn't forgetful, I wasn't uninterested, I had a real medical condition," she says.

She volunteered to be tested by a professor researching prosopagnosis at University College London. A series of tests confirmed that Mary Ann was indeed prosopagnosic.

"It was a relief to be officially told that the problem was not my fault," she says. Shortly afterwards Evie, David and Felicity Ann were also diagnosed with the same condition.

Felicity Ann, 80, was delighted to have a diagnosis after so many years and now suspects her father had the condition too.

She says: "Back in my childhood people weren't interested in a problem unless you were in pain.

"It was an enormous effort to try and hide that I didn't know who I was talking to, especially at work.

"Even now I find it easier to smile at everybody I meet, that way I can't offend somebody by not knowing them."

But luckily nobody in the family has the most severe form of the condition, which leaves sufferers unable to identify members of their own family or even themselves in the mirror.

Scientists are still trying to discover why the area of the brain that processes faces has not developed in prosopagnosics.

But, knowing they have a neurological problem is enough for the family as they can now confidently explain away why they can't recognise a friend or colleague.

David, who is less severely affected, still prefers to bluff his way through introductions as explaining the unusual condition is simply too complicated.

But Mary Ann says: "At last I can tell people I'm not being rude and ask them not to be offended, though sadly some still are. I find this particularly difficult. Perhaps we will take up Evie's T-shirt idea after all."

My Brilliant Brain: Make Me A Genius, is on National Geographic Channel this Sunday at 8pm.